Polyolefin microporous films are broadly used as separation and permselective separation films, separation materials and the like for various types of substances, and examples of applications thereof include microfiltration films, separators for fuel cells, separators for capacitors, base materials of functional films to develop new functions by filling pores with a functional material, and separators for batteries. Particularly, polyolefin microporous films are suitably used as separators for lithium ion batteries broadly used for mobile devices, such as laptop computers, cellular phones, digital cameras and the like. For the reason, it can be cited that the mechanical strength and insulative performance of the films are high.
For example, Patent Literatures 1 and 2 disclose polyethylene microporous films having both good permeation performance and high strength. Patent Literature 3 discloses a polyolefin microporous film whose thermal shrinkage is suppressed. Patent Literature 4 discloses a polyolefin microporous film having a narrow pore distribution and a high strength.
Patent Literature 5 further discloses a rolled microporous film excellent in workability in production of a battery.